1. Field of the Invention
The present invention relates to an engine control system for a construction machine, and more particularly to an engine control system for a construction machine such as a hydraulic excavator wherein a diesel engine having an electronic fuel injection device (electronic control governor) is used as a prime mover.
2. Description of the Related Art
A construction machine such as a hydraulic excavator generally includes at least one hydraulic pump for driving a plurality of actuators, and a diesel engine is used as a prime mover for rotatively driving the hydraulic pump. The diesel engine is controlled in injected fuel amount and fuel injection timing by a fuel injection device. With recent development of electronic control in the fuel injection device, in particular, a fuel injection rate has also become freely controllable in addition to the injected fuel amount and the fuel injection timing. As a result, good combustion is realized and engine performance is improved in a wide range.
In a fuel injection device for a diesel engine disclosed in JP, A, 1-121560, for example, a valve opening pressure is controlled such that it is lowered to stabilize an injection rate in a low-speed, low-load region, and is raised to increase the injection rate and shorten an injection period for avoiding the generation of black smoke in a low-speed, high-load region.
Further, the fuel injection timing has become freely controllable by determining the optimum injection timing depending on a status variable such as related to engine revolution, which also contributes to achieving good combustion.
Earlier timing of fuel injection provides a higher combustion temperature of fuel injected into a cylinder and hence better fuel efficiency (fuel consumption). As stated in, e.g., "Mechanization of Construction" (1996 DECEMBER No. 562), an article titled "Overview and Inspection/Servicing of Diesel Engine Adapted for Exhaust Gas Regulation (No. 2)", page 63, however, NO.sub.x meaning NO and NO.sub.2 together, which are said to be responsible for photochemical smog, generally tends to be produced during high-speed, high-load operation. To make exhaust gas clean, therefore, a method of delaying the fuel injection timing during the high-speed, high-load operation, where NO.sub.x tends to be produced, is employed.